Pulsed electric machine control

ABSTRACT

A variety of methods, controllers and electric machine systems are described that facilitate pulsed control of electric machines (e.g., electric motors and generators) to improve the machine&#39;s energy conversion efficiency. Under selected operating conditions, the electric machine is intermittently driven (pulsed). The pulsed operation causes the output of the electric machine to alternate between a first output level and a second output level that is lower than the first output level. The output levels are selected such that at least one of the electric machine and a system that includes the electric machine has a higher energy conversion efficiency during the pulsed operation than the electric machine would have when operated at a third output level that would be required to drive the electric machine in a continuous manner to deliver the desired output. In some embodiments, the second output level is zero torque.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/544,446, filed on Dec. 7, 2021, which is a Continuation of U.S.application Ser. No. 16/912,313, filed on Jun. 25, 2020 (now U.S. Pat.No. 11,228,272, issued Jan. 18, 2022) which is a Continuation of U.S.application Ser. No. 16/353,166, filed on Mar. 14, 2019 (now U.S. Pat.No. 10,742,155, issued Aug. 11, 2020), which claims priority of U.S.Provisional Patent Application Nos. 62/644,912, filed on Mar. 19, 2018;62/658,739, filed on Apr. 17, 2018; and 62/810,861 filed on Feb. 26,2019, all of which are incorporated herein by reference in theirentirety.

BACKGROUND

The present application relates generally to electric machine control.More specifically, control schemes and controller designs are describedthat pulse the operation of an electric machine during selectedoperating conditions to facilitate operating the electric machine in amore energy efficient manner.

The phrase “electric machine” as used herein is intended to be broadlyconstrued to mean both electric motors and generators. Electric motorsand generators are structurally very similar. When an electric machineis operating as motor, it converts electrical energy into mechanicalenergy. When operating as a generator, the electric machine convertsmechanical energy into electrical energy.

Electric motors and generators are used in a very wide variety ofapplications and under a wide variety of operating conditions. Ingeneral, many modern electric machines have relatively high energyconversion efficiencies. However, the energy conversion efficiency ofmost electric machines can vary considerably based on their operationalload. Many applications require that the electric machine operate undera wide variety of different operating load conditions, which means thatthe electric machine often doesn't operate as efficiently as it iscapable of. The nature of this problem is illustrated in FIG. 1 , whichis a motor efficiency map 10 that diagrammatically shows the efficiencyof a representative motor under different operating conditions. Morespecifically, the figure plots the energy conversion efficiency of themotor as a function of motor speed (the X-axis) and torque generated(the Y-axis).

As can be seen in FIG. 1 , the illustrated motor is generally mostefficient when it is operating within a particular speed range andgenerating torque within a defined range. For the particular motorshown, the most efficient region of its operating range is the operatingregion labeled 14 which is generally in the range of 4500-6000 RPM witha torque output in the range of about 40-70 Nm where its energyconversion efficiency is approximately 96%. The region 14 is sometimesreferred to herein as the “sweet spot”, which is simply the motor's mostefficient operating region.

As can be seen in FIG. 1 , at any particular motor speed, there will bea corresponding most efficient output torque which is diagrammaticallyillustrated by maximum efficiency curve 16. For any given motor speed,the motor's efficiency tends to drop off somewhat when the motor's loadis higher or lower than the most efficient load. In some regions themotor's efficiency tends to drop relatively quickly, as for example whenthe torque output falls below about 30 Nm in the illustrated motor.

If the operating conditions could be controlled so that the motor isalmost always operated at or near its sweet spot, the energy conversionefficiency of the motor would be quite good. However, many applicationsrequire that the motor operate over a wide variety of load conditionswith widely varying torque requirements and widely varying motor speeds.One such application that is easy to visualize is automotive and othervehicle or mobility applications where the motor speed may vary betweenzero when the vehicle is stopped to a relatively high RPM when cruisingat highway speeds. The torque requirements may also vary widely at anyof those speeds based on factors such as whether the vehicle isaccelerating or decelerating, going uphill, downhill, going onrelatively flat terrain, etc., the weight of the vehicle and many otherfactors. Of course, motors used in other applications may be subjectedto a wide variety of operating conditions as well.

Although the energy conversion efficiency of conventional electricmachines is generally good, there are continuing efforts to furtherimprove energy conversion efficiencies over broader ranges of operatingconditions.

SUMMARY

A variety of methods, controllers and electric machine systems aredescribed that facilitate pulsed control of electric machines (e.g.,electric motors and generators) to improve the energy conversionefficiency of the electric machine when operating conditions warrant.More specifically, under selected operating conditions, an electricmachine is intermittently driven (pulsed). The pulsed operation of theelectric machine causes the output of the electric machine to alternatebetween a first output level and a second output level that is lowerthan the first output level. The first and second output levels areselected such that at least one of the electric machine and a systemthat includes the electric machine has a higher energy conversionefficiency during the pulsed operation than the electric machine wouldhave when operated at a third output level that would be required todrive the electric machine in a continuous manner to deliver the desiredoutput. In some embodiments, the second output level is zero torque (orsubstantially zero torque).

In some embodiments, the electric machine is driven in a pulsed mannerwhen a desired output is less than a designated output level for a givenmotor speed and driven in a continuous manner when the desired motoroutput is greater than or equal to the designated output level.

In some embodiments, a power converter is used to control the output ofthe electric machine. Depending on the application, the power convertermay take the form of an inverter, a rectifier, or other appropriatepower converter.

The frequency of the pulsing may vary widely with the requirements ofany particular application. By way of examples, in various embodimentsthe electric machine alternates between the first and second outputlevels at least 10, 100 or 1000 times per second.

In some embodiments, a sigma delta converter is used to control thepulsing of the electric machine. A wide variety of different sigma deltaconverter architectures may be used. In some embodiments, the sigmadelta converter is a first order sigma delta converter. In others, athird order sigma delta converter is used. In still others, higher ordersigma delta converters may be used. The sigma delta converters may beimplemented algorithmically, digitally, using analog components and/orusing hybrid approaches.

In other embodiments, a pulse width modulation controller is used tocontrol the pulsing of the electric machine.

In some embodiments, the first output level varies in accordance withvariations in the current operating speed of the electric machine. Invarious embodiments, the first output level may correspond to anelectric machine output level that is or is close to the highest systemor electric machine energy conversion efficiency at a current operatingspeed of the electric machine. In some embodiments, a duty cycle of thepulsing varies in accordance with variations in the desired output.

Machine controllers and electric machine systems are described forimplementing all of the functionalities described above. In variousembodiments, the system may be configured to operate as a motor, agenerator, or as a motor/generator.

In various embodiments, the electric machine may be: an inductionmachine; a switched reluctance electric machine; a synchronous ACelectric machine; a synchronous reluctance machine; a permanent magnetsynchronous reluctance machine; a hybrid permanent magnet synchronousreluctance machine; an externally excited AC synchronous machine (alsoreferred to in the art by terms such as an electrically excitedsynchronous machine (EESM), an externally excited synchronous machine(EESM), or a wound field synchronous machine (WFSM); a permanent magnetsynchronous machine; a brushless DC electric machine; an electricallyexcited DC electric machine; a permanent magnet DC electric machine; aseries wound DC electric machine; a shunt DC electric machine; a brushedDC electric machine; a compound DC electric machine; an eddy currentmachine; an AC linear machine; an AC or DC mechanically commutatedmachine; or an axial flux machine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a representative Torque/Speed/Efficiency graph illustratingthe energy conversion efficiency of a representative electric motorunder different operating conditions.

FIG. 2A is a graph illustrating a pulsed drive signal for an electricmachine.

FIG. 2B is a diagrammatic representation of a continuous three-phase ACdrive signal waveform.

FIGS. 2C and 2D are pulsed three-phase AC waveforms having a 50% dutycycle that represent the same average power as the continuous waveformof FIG. 2B.

FIG. 3 is a functional block diagram that diagrammatically illustratesan electric machine control architecture in accordance with onedescribed embodiment.

FIG. 4 is a flow chart illustrating a motor control scheme in accordancewith another embodiment.

FIG. 5 is a representative Torque/Speed/Efficiency graph illustratingthe energy conversion efficiency of a representative inverter underdifferent operating conditions.

FIG. 6 is graph illustrating the combined energy conversion efficiencyof a representative inverter/electric motor combination.

FIG. 7 is a functional block diagram that diagrammatically illustrates amotor controller architecture that includes a sigma-delta based pulsegenerator in accordance with another embodiment.

FIG. 8 is a diagrammatic functional block diagram of a first ordersigma-delta converter.

FIG. 9A is a graph diagrammatically representing the desired power riseand fall for pulsed power.

FIG. 9B is a graph diagrammatically representing the actual power riseand fall that may be seen when a conventional motor is pulsed in thedescribed manner.

FIG. 10 is a Torque/Speed/Efficiency graph showing the distribution ofdrive points representing the simulated output of an automotive electricmotor during an FTP 75 drive cycle.

FIG. 11 is a diagrammatic block diagram of an electric motor/generatorhaving transient control circuit configured to shorten themotor/generator on/off rise and fall times in accordance with anembodiment.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present disclosure relates generally to pulsed control of electricmachines (e.g., electric motors and generators) that would otherwise beoperated in a continuous manner to improve the energy conversionefficiency of the electric machine when operating conditions warrant.More specifically, under selected operating conditions, an electricmachine is intermittently driven (pulsed) at more efficient energyconversion operating levels to deliver a desired average torque moreenergy efficiently than would be attained by traditional continuousmotor control.

Many types of electrical machines, including mechanically commutatedmachines, electronically commutated machines, externally commutatedasynchronous machines, and externally commutated synchronous machinesare traditionally driven by a continuous, albeit potentially varying,drive current when the machine is used as a motor to deliver a desiredtorque output. The drive current is frequently controlled by controllingthe output voltage of a power converter (e.g., an inverter) which servesas the voltage input to the motor. Conversely, the power output of manytypes of generators is controlled by controlling the strength of amagnetic field—which may, for example, be accomplished by controlling anexcitation current supplied to rotor coils by an exciter. (The excitermay be part of a rectifier or other suitable component). Regardless ofthe type of machine, the drive current for a motor, or the currentoutput by a generator, tends to be continuous.

With pulsed control, the output of the machine is intelligently andintermittently modulated between “torque on” and “zero (no) torque”states in a manner that: (1) meet operational demands, while (2)improving overall efficiency. Stated differently, under selectedoperating conditions, the electric machine is intermittently driven at amore efficient energy conversion operating level (the “torque on” state)to deliver a desired output. In the periods between the pulses, themachine ideally does not generate or consume any torque (the “zerotorque” state). This can conceptually be thought of as turning theelectric machine “off.” In some implementations, this can beaccomplished by effectively turning the electric machine “off,” as forexample, by shutting off drive current to a motor or the excitationcurrent for a generator. However, in other implementations, the electricmachine may be controlled during the “zero torque” state in a mannerthat attempts to cause the torque generated by the electric machine tobe zero or as close to zero as may be practical or appropriate for theparticular machine. In some implementations, any power converters usedin conjunction with the electric machine may effectively be turned offfor at least portions of the “zero torque” periods as well.

As discussed in the background, FIG. 1 illustrates the energy conversionefficiency of a representative motor. The map illustrated in FIG. 1 isthe efficiency map for an internal permanent magnet synchronous motorused in a 2010 Toyota Prius. It should be understood that this map ismerely illustrative. Similar efficiency maps can be generated for justabout any electric machine although the characteristics of the map willvary with the machine that is characterized.

As can be seen in FIG. 1 , at any particular motor speed, there will bea corresponding most efficient output torque which is diagrammaticallyillustrated by maximum efficiency curve 16. From a conceptualstandpoint, when the desired motor torque is below the most efficientoutput torque for the current motor speed, the overall efficiency of themotor can be improved by pulsing the motor. Conversely, when the desiredmotor torque is at or above the maximum efficiency curve 16, the motormay be operated in a conventional (continuous/non-pulsed) manner todeliver the desired torque.

FIG. 2A illustrates an example of pulsed motor operation. In thisparticular example, the desired motor torque is 10 Nm, but the mostefficient torque output for the current operating motor speed is 50 Nm.Conceptually, the motor can be driven to deliver a net torque of 10 Nmby causing the motor to deliver 50 Nm of torque for 20% of the time andthen delivering no (zero) torque the remaining 80% of the time. Sincethe motor operates more efficiently when it is delivering 50 Nm thanwhen it delivers 10 Nm, the motor's overall efficiency can be improvedby pulsing the motor's operation in the described manner. In the exampleillustrated in FIG. 2A the motor produces a motor output of 50 Nm(labeled 24) for a period of 1 time unit out of every 5 time units andthen the motor is controlled to produce zero torque during theintervening 4 time units.

As long as the desired motor output doesn't exceed 50 Nm, the desiredmotor output can theoretically be met merely by changing the duty cycleof the motor operating at 50 Nm. For example if the desired motor outputchanges to 20 Nm, the duty cycle of the motor operating at 50 Nm can beincreased to 40%; if the desired motor output changes to 40 Nm, the dutycycle can be increase to 80%; if the desired motor output changes to 5Nm, the duty cycle can be reduced to 10% and so on. More generally,pulsing the motor can potentially be used advantageously any time thatthe desired motor torque falls below the maximum efficiency curve 16.

The scale of the time units actually used may vary widely based on thesize, nature and design needs of any particular system. In practice,when the motor is switched from the “torque on” to “zero torque” statesrelatively rapidly to achieve the designated duty cycle, the fact thatthe motor is actually being switched back and forth between these statesmay not materially degrade the motor's performance from an operationalstandpoint. In some embodiments, the scale of the periods for eachon/off cycle is expected to be on the order of 100 μsec to 0.10 seconds(i.e. pulsing at a frequency in the range of 10 to 10,000 Hz), as forexample in the range of 20 to 1000 Hz, or 20 to 100 Hz as will bediscussed in more detail below.

The zero torque portions of the pulse cycle might conceptually be viewedas shutting the motor off—although in many cases the motor may notactually be shut off during those periods or may be shut off for onlyportions of the “zero torque” intervals.

Many electric machines are designed to operate using alternatingcurrent. FIGS. 2B-2D are plots illustrating the difference betweencontinuous and pulsed alternating currents that may be input to anelectric machine operating as a motor—as for example, a three-phaseinduction motor. In each plot, current is plotted on the vertical axisand time is plotted along the horizontal axis.

FIG. 2B illustrates conventional sinusoidal three-phased input current42 a, 42 b, and 42 c delivered to the electric machine. Phase B, denotedby curve 42 b leads phase A, denoted by 42 a by 120 degrees. Phase C,denoted by curve 42 c, leads phase B by 120 degrees. The sine waveperiod is □. The three-phased input power 42 is continuous (not pulsed)and has a designated maximum amplitude of approximately 50 amps. Itshould be appreciated that 50 amps is only a representative maximumcurrent and the maximum current may have any value.

FIGS. 2C and 2D illustrate two examples of different pulsed three-phasedcurrents 44 a, 44 b, and 44 c and 46 a, 46 b, and 46 c that each have a50% duty cycle and a peak amplitude of approximately 100 amps. As inFIG. 2B the period of the base sine wave is □, however, now the sinewave is modulated on and off. Assuming the motor speed is the same andthe generated torque is substantially proportional to current, as isoften the case, the delivered current in FIGS. 2C and 2D produces thesame average torque as the continuously applied three-phased inputcurrent of FIG. 2B. The difference between pulsed currents 44 a-c and 46a-c is the duration of their respective current pulses and theinterleaved “off” periods. In FIG. 2C, the current pulses 44 a-c areinterleaved with “off” periods of equal length. The length of each onand off period is 2□. In FIG. 2D, the current pulses 46 a-c and theinterleaved “off” periods again have equal duration. In this case theduration is □/2. In both examples, the duty cycle is 50%. However, theduration of the “on” and “off” time durations is different, i.e. thefrequency of the pulsed modulation is different. The frequency of thepulsed modulation may vary based on the type of electrical machine used,noise and vibration considerations, current operating rotor speed andother factors.

FIGS. 2C-2D illustrate applications in which the “on” motor drive pulsesare evenly spaced while the motor is operated at a steady state desiredoutput level. Such an approach works well in many circumstances, but isnot a requirement. The duty cycle need not be 50%, but can be adjustedto match the desired average output torque. Also, the phase of theon/off pulses need not be synchronized with the phase of the applied ACpower. Thus, the relative sizes and/or timing of the motor drive pulsescan be varied as long as they average out to deliver the desired averagetorque.

Power Converters and System Efficiency

There are a wide variety of different electric machines and each machinehas its own unique efficiency characteristics. Further, at differentoperating speeds, the electric machine will have different efficiencycurves as should be apparent from a cursory review of FIG. 1 .Therefore, the operating regions in which pulsed control can provideefficiency gains will vary significantly based on factors including theparticular electric machine's characteristics and the currentoperational rotor speed.

When AC electric machines are used in conjunction with a battery orother DC power source/sink (store), power converters (e.g. inverters andrectifiers) will typically be used to convert between DC and AC power.For example, inverters are used to convert power received from a DCpower supply, such as a battery or capacitor, into AC input powerapplied to a motor. Conversely, rectifiers are used to convert AC powerreceived from an electric machine operating as a generator into DCoutput power. Some power converters may function as either an inverteror a rectifier depending upon whether the electric machine isfunctioning as a motor or a generator.

The energy conversion efficiency of power converters will also typicallyvary over the operating range of the converter. For example, FIG. 5 isunderstood to be an inverter energy conversion efficiency map for a 2010Toyota Prius inverter (i.e., the inverter for the motor represented inFIG. 1 ). Although the energy conversion efficiency of the illustratedinverter is quite good, its efficiency also drops off noticeably incertain regions—most notably at lower torque outputs and at lower motorspeeds. Thus, when optimizing control of a motor that is part of aninverter/electric motor combination, it is desirable to consider theenergy conversion efficiency of the overall inverter/electric motorsystem as opposed to the energy conversion efficiency of the motoralone. Similarly, when optimizing the control of a generator that ispart of a rectifier/generator system, it is desirable to consider theenergy conversion efficiency of the overall rectifier/generator systemas opposed to the energy conversion efficiency of the generator alone.

Preferably, the pulsed control of an electric machine will be modeled toaccount for the efficiencies of any/all of the components that influencethe energy conversion during pulsing. For example, when power for an ACelectric motor is drawn from a battery, the battery's power deliveryefficiency, cabling losses between components and any other loss factorscan be considered in addition to the inverter and motor efficiencies,when determining the motor drive signal that delivers the best energyconversion efficiency.

In general, the overall energy conversion efficiency of a powerconverter/electric machine system is a function of the product of theconverter conversion efficiency times the electric machine conversionefficiency times the delivery efficiency of other components. Thus, itshould be appreciated that the parameters of the pulsed drive signalthat has the maximum system energy conversion efficiency may bedifferent than the parameters that would provide the best energyconversion efficiency for the motor itself.

FIG. 6 is a representative energy conversion efficiency map for acombined inverter/AC electric motor. More specifically, FIG. 6 shows thecombined efficiency of a Tesla inverter/motor propulsion system. Sincethe inverter and the electric motor work together, the best overallelectric motor system energy conversion efficiency can be optimized byselecting the pulse voltage and corresponding duty cycle based on thecombined system energy conversion efficiency map rather than using theelectric motor's energy conversion efficiency map alone.

Often, the energy conversion efficiency map for a particular electricmachine system (e.g. a combined power converter/electric machine;battery/power converter/electric machine; etc.) will be more complexthan the efficiency map for the electric machine itself. As such, theremay be local efficiency peaks above the maximum efficiency curve. Thatis, there may be a region of the energy conversion efficiency map whereat a given motor speed, operation at a particular torque output that isabove the “maximum” possible efficiency for operation at that motorspeed may be more efficient than a range of intermediary torque outputsthat are above the maximum efficiency curve, but below that particulartorque output. One such region is designated 61 in FIG. 6 . It should beappreciated that when the energy conversion efficiency map has this typeof topography, pulsed operation at the local efficiency peak may be moreefficient than continuous operation at those intermediary output levels.In such circumstances, the motor can be pulsed at the level of the localefficiency peak to deliver such intermediary torque outputs.

Pulsed System Control

FIG. 3 illustrates a control architecture suitable for controlling anelectric machine in the described manner. In this embodiment, the system100 includes a machine controller 110, a pulse controller (pulsegenerator) 120, a power supply/sink 130, a power controller/converter140, and an electric machine 160. The pulse controller 120 isresponsible for controlling/directing the timing of the pulsing ofelectric machine 160 when pulsed operation is called for. In theembodiment illustrated in FIG. 3 , the pulse controller is shown as acomponent that is separate from the machine controller 110 to facilitateexplanation of its function. However, in various embodiments, the pulsecontroller may be implemented as part of the machine controller 110, asa separate component, as part of power controller/converter 140 or inother appropriate forms.

When the electric machine 160 is operated as a motor, the machinecontroller functions as a motor controller, and the powercontroller/converter 140 is responsible for converting power 132received from power supply 130 to a form that is suitable for drivingthe motor 160. Conversely, when the machine 160 is operated as agenerator, the machine controller 110 functions as a generatorcontroller and the power controller/converter 140 converts powerreceived from the generator to a form suitable for delivery to the powersink 130. In embodiments in which the power supply/sink can supply orreceive power directly in the form required by or outputted by theelectric machine, the power controller 140 can conceptually take theform of a switch or logical multiplier that simply turns the motor onand off to facilitate the desired pulsing.

The power supply/sink 130 can take any suitable form. In someimplementations, the power supply/sink may take the form of a battery ora capacitor. In other implementations, the source may be a power grid(e.g., “wall power”), a photovoltaic system, or any other availablesource. Similarly, the sink may be an electrical load (such as anelectrically operated machine or appliance, a building, a factory, ahome, etc.), a power grid or any other system that uses or storeselectrical power.

The power controller/converter 140 can also take a wide variety ofdifferent forms. When the power supply/sink 130 is a DC power supply andthe electric machine 160 is an AC motor, the power controller/converter140 can take the form of an inverter. Conversely, when the powersupply/sink 130 is a DC power sink and the electric machine 160 is an ACgenerator, the power controller/converter 140 can take the form of arectifier. When both the power supply/sink 130 and the electric machineare AC components, the power controller/converter 140 may include abidirectional or 4 quadrant power converter.

In FIG. 3 , the requested output is labeled 113, the torque delivered orreceived by the electric machine is labeled 161 and the motor/generatorspeed is labeled 164. In some embodiments, the machine controller 110includes a data structure 115 (as for example a lookup table) thatserves as a pulsed operation map that defines the operating region inwhich pulsed motor control is desired and/or appropriate as well as thespecific duty cycles that are appropriate for specific operatingconditions.

Once the desired duty cycle is determined, the duration and nature ofthe pulses used to drive the motor can be determined/generated in a widevariety of manners. As will be described in more detail below, onerelatively simple approach is to use a pulse width modulation (PWM)controller as the pulse controller 120.

In FIG. 3 , logical multiplier 123 is shown as multiplying a pulsedcontrol signal 124 times a power level signal 119 output by machinecontroller 110 to create a power converter control signal 128. It shouldbe appreciated that the logical multiplier 123 is shown for the purposesof explanation and in practice, the function of the multiplier 123 canbe accomplished by the machine controller 110, by the power converter140, or in other suitable manners. For example, in some embodiments, themachine controller 110 may simply set the output of the power converter140 to zero during the “off” phases of the duty cycle and to the desiredoperational output level (e.g., the most efficient output level for thecurrent machine speed) during the “on” phases of the duty cycle.

FIG. 4 illustrates a control flow that may be performed by machinecontroller 110 to cause the electric machine 160 to efficiently delivera desired torque. To simplify the discussion, an embodiment in which theelectric machine 160 functions as a motor is described. In thisarrangement, the power supply/sink 130 acts as a power supply and themachine controller 110 functions as a motor controller.

Initially, the motor controller 110 receives the currently requestedmotor output 113 and any required motor state information such as thecurrent motor speed 164 as represented by block 171. The motorcontroller 110 then determines whether the requested output is withinthe pulsed control range as represented by decision block 172. Thisdecision can be made in any desired manner. By way of example, in someembodiments, a look-up table 115 or other suitable data structure can beused to determine whether pulsed control is appropriate. In someimplementations a simple lookup table may identify a maximum torquelevel at which pulsed control is appropriate for various motor speeds.In such an implementation, the current motor speed may be used as anindex to the lookup table to obtain a maximum torque level at which thepulsed control is appropriate under the current operating conditions.The retrieved maximum torque value can then be compared to the requestedtorque to determine whether the requested output is within the pulsecontrol range.

In other embodiments, the lookup table 115 may provide additionalinformation such as the desired duty cycle for pulsed operation based onthe current operating conditions. In one such implementation, the motorspeed and the torque request may be used as indices for a lookup tablewith each entry in the lookup table indicating the desired duty cyclewith interpolation being used to determine an operational duty cyclewhen the actual torque and/or motor speeds are between the index valuesrepresented in the table.

If the requested torque/current operating conditions are outside of thepulsed control range for any reason, then traditional (i.e.continuous/non-pulsed) motor control is used as represented by the “no”branch flowing from block 172. As such, pulsing is not used and thepower converter 140 is directed to deliver power to the motor 160 at alevel suitable for driving the motor to deliver the requested output 113in a conventional manner as represented by block 174. Conversely, whenthe requested torque/current operating conditions are within the pulsedcontrol range, then pulsed control is utilized as represented by the“yes” branch flowing from block 172. In such embodiments, the motorcontroller 110 will direct the power converter 140 to deliver power tothe motor in a pulsed manner. During the “on” pulses, the powerconverter 140 is directed to deliver power at a preferred outputlevel—which would typically (but not necessarily) be at or close to themaximum efficiency operating level for the current motor speed. Duringthe “off” pulses, the motor ideally outputs zero torque. In someembodiments, the timing of the pulsing is controlled by pulse controller120 as will be discussed in more detail below.

To facilitate pulsed operation, the motor controller 110 determines thedesired output level (block 175) and the desired duty cycle (block 176)for pulsed operation at the current motor speed (which is preferably ator close to the system's maximum efficiency energy conversion outputlevel at the current motor speed—although other energy efficient levelscan be used as appropriate). The motor controller and the pulsecontroller then direct the power converter to implement the desired dutycycle (block 178) at the designated power level. Conceptually, this maybe accomplished by effectively turning the power supply on and off at arelatively high frequency such that the fraction of the time that poweris supplied to the motor corresponds to the desired duty cycle, and thepower level corresponds to the preferred output level. In someembodiments, the “off” portion of the duty cycle may be implemented bydirecting the power controller/converter 140 to drive the motor todeliver zero torque.

The frequency at which the power is pulsed is preferably determined bythe machine controller 110 or the pulse controller 120. In someembodiments, the pulsing frequency can be fixed for all operation of themotor, while in others it may vary based on operational conditions suchas motor speed, torque requirements, etc. For example, in someembodiments, the pulsing frequency can be determined through the use ofa look-up table. In such embodiments, the appropriate pulsing frequencyfor current motor operating conditions can be looked up usingappropriate indices such as motors speed, torque requirement, etc. Inother embodiments, the pulsing frequency is not necessarily fixed forany given operating conditions and may vary as dictated by the pulsecontroller 120. This type of variation is common when using sigma deltaconversion in the determination of the pulses as discussed below. Insome specific embodiments, the pulsing frequency may vary proportionallyas a function of motor speed, at least in some operating regions of themotor.

Although FIG. 4 illustrates some of the steps sequentially to facilitatea clear understanding of the functionality provided, it should beunderstood that many of the steps can be combined and/or reordered inpractice. For example, the entries in a multi-dimensional lookup table115 that uses requested output 113 and current electric motor speed 164as indices may indicate both the preferred output level and the dutycycle that is appropriate for current operation.

In some embodiments, a value stored in the lookup table (such as a dutycycle of 1 (100%) or other suitable wildcards) can optionally be used toindicate that pulsing is not desired. Of course a wide variety of otherconventions and data structures can be used to provide the sameinformation.

In some embodiments, the pulsed control table can be incorporated into alarger table that defines operation at all levels such that theoperational flow is the same regardless whether conventional or pulsedcontrol is desired with the conventional control merely being defined bya duty cycle of 1 and the appropriate motor input power level, and thepulsed control being defined by a smaller duty cycle and use of thepreferred motor input power level.

In some embodiments, it may be desirable to avoid the use of pulsing insome operating regions even when efficiency improvements are possible,based on other considerations. As will be discussed in more detailbelow, these other considerations may be based on factors such a noiseand vibration, the practical switching capabilities of the controller,etc.

The machine controller described herein may be implemented in a widevariety of different manners including using software or firmwareexecuted on a processing unit such as a microprocessor, usingprogrammable logic, using application specific integrated circuits(ASICs), using discrete logic, etc. and/or using any combination of theforegoing.

It is notable that in many circumstances, existing electric machines andmachine controllers can readily be retrofitted to obtain the describedbenefits. For example, many machine controllers are implemented usingsoftware or firmware executed on a processing unit which already hasaccess to control input parameters suitable for use in the describedcontrol (e.g., a requested motor output and a current motor speed). Insuch cases, it may be possible to obtain noticeable efficiencyimprovements by installing a relatively simple software update.

Pulse Generation

As suggested above, once the desired duty cycle is determined, theduration and nature of the pulses used to drive the motor can bedetermined/generated in a wide variety of manners. One relatively simpleapproach is to use a pulse width modulation (PWM) controller as thepulse controller 120.

It is noted that pulse width modulation is commonly used in certaintypes of motor control, including AC electric motor control and DCbrushless motor control, but such pulse width modulation is used at avery different location in the control scheme. Specifically, when an ACinduction motor is powered by a battery (which provides DC power) aninverter is typically used to facilitate the conversion of DC power toAC power. Commonly, a PWM controller (not shown) is used as part of theinverter controller to control the amplitude of the AC signal that isgenerated by the inverter. Continuous AC power generated by the inverteris then supplied to the electric motor at the desired frequency andamplitude. PWM controllers are similarly used in brushless DC motors tocontrol the amplitude of the continuous signal that is supplied to themotor.

The pulsed power utilized herein is quite different. Specifically, powerconverter 140 is controlled to cyclically switch between producing ahigh efficiency torque output (e.g. the peak efficiency torque) and notorque in the electric machine 160 as discussed above with reference toFIGS. 2A-2D. In an induction motor, this results in the magnetic flux inthe motor windings effectively dropping to zero.

Although traditional pulse width modulation will work in manyapplications, a potential drawback is the possibility of the pulsinggenerating undesirable vibrations or noise as the motor and/or powersupply are turned on and off. Steady state operation of the motor at thesame pulse cycle for a period of time is particularly susceptible togenerating such vibration. There are a number of ways to mitigate suchrisks including some that will be described in more detail below.Another approach is to add some dither to the commanded pulse cycle.

As suggested above, the period for each cycle during pulsed operation(or inversely the pulsing frequency) may vary widely based on the designneeds and the nature of the controlled system ranging from microsecondsto tenths of a second or longer. A variety of factors will influence thechoice of the cycle period. These include factors such as thecapabilities and characteristics of the motor, the transitory effectsassociated with switching, potential NVH (noise, vibration andharshness) concerns, the expected operational loads, etc. In general,the pulsing frequency selected for any particular application willinvolve a tradeoff including factors such as NVH considerations,required responsiveness of the electric machine, efficiency lossassociated with pulsing, etc. For example, in some automotiveapplications, pulsing frequencies on the order of 20 Hz-1000 Hz arebelieved to work well.

Sigma Delta Control

Referring next to FIG. 7 , another embodiment of the pulse generatorwill be described. The illustrated architecture is similar to thearchitecture illustrated in FIG. 3 except that in this embodiment, asigma-delta converter 190, is used as the pulse generator 120. As willbe appreciated by those familiar with sigma delta control, acharacteristic of sigma delta control is that it facilitates noiseshaping and tends to reduce/eliminate idle tones and push noise tohigher frequencies. When noise is randomized and/or spread tofrequencies that are above the limits of human perception, it is less ofa concern since any such noise and/or vibration is not bothersome to theusers of the motor. Therefore, in the context of an automotive electricmotor application, use of sigma delta control tends to reduce thelikelihood of vehicle occupants perceiving noise or vibrations due tothe pulsed motor control.

A wide variety of different sigma delta converters may be used as sigmadelta converter 190 and the sigma delta converters may utilize a varietyof different feedback schemes. By way of example, first order sigmadelta conversion works well. One particularly desirable feature of usinga first order sigma delta converter is that the controller is inherentlystable. Although a first order sigma delta converter works well, itshould be appreciated that in other embodiments, higher order sigmadelta converters may be used (e.g., sigma delta converters that utilizea higher number of integrators than a first order sigma deltaconverter). For example, third order sigma delta converters (as forexample converters using the Richie architecture) or higher order sigmadelta converters may be used.

Generally, the sigma delta converters may be implementedalgorithmically, digitally, using analog components and/or using hybridapproaches. For example, in various embodiments, the sigma deltaconverter may be implemented on a processor, on programmable logic suchas an FPGA, in circuitry such as an ASIC, on a digital signal processor(DSP), using analog, digital and/or hybrid components, or any/or usingother suitable combinations of hardware and/or software. In variousembodiments, the sigma delta controller may utilize sample data sigmadelta, continuous time sigma delta, differential sigma delta, or anyother suitable sigma delta implementation scheme.

U.S. Pat. No. 8,099,224 and U.S. Patent Publication No. 2018-0216551,which are incorporated herein by reference in their entirety, describe anumber of representative sigma delta converter designs. Although theapplications described therein are for controlling different types ofpower plants, similar types of converters may be used for the presentapplication.

Referring next to FIG. 8 , a representative first order sigma-deltaconverter 200 will be described. The first order sigma-delta converter200 includes a difference amplifier 201, an integrator 203, and acomparator 205. The difference amplifier 201 amplifies the differencebetween an input signal 209 and a feedback signal 212 and outputs adifference signal 216 which is fed to integrator 203. Integrator 203integrates the difference signal and outputs an integrator output signal217 which is fed to comparator 205. The comparator 205 acts as a one-bitquantizer and outputs a pulsed (high/low) digital control signal 220that is representative of the input signal 209. The one-bit controlsignal 220 outputted from the comparator 216 is generated by comparingthe output of the integrator 203 with a reference voltage. The output iseffectively a string of ones and zeros that is outputted at thefrequency of the sigma delta converter's clock. The low signal istreated as a request for zero power from the power supply and the highsignal is treated as a request for the most efficient (or otherdesignated) power level for the current motor speed.

Generally, in order to ensure high quality control, it is desirable thatthe clock signal 226 for the sigma delta converter (and thus the outputstream of the comparator 205) have a frequency that is many times theexpected frequency of the rate of change of the input signal 209, toprovide good resolution and oversampling of the input signal. Ingeneral, clock frequencies on the order of 100 kHz to 1 MHz or higherwork well for automotive type applications where the input signal (whichis generally based on the driver's torque request—e.g. the acceleratorpedal) tends to vary at rates of less than 5 Hz. That is, the output ofthe comparator 205 is sampled at a rate of at least 100 kHz-1 MHz(although both higher and lower sampling rates may be used in variousembodiments). The clock signal 226 provided to the comparator 216 maycome from any suitable source. For example, in some embodiments, theclock signal 226 is provided by a crystal oscillator.

In various embodiments, the comparator 205 can be configured to enforcedesired constraints on the pulsing (which is sometimes referred toherein as performing as a functionally intelligent comparator). In asimple example, the comparator can be constrained to define minimumand/or maximum “on” times, minimum (and/or maximum) “off” times etc. aswill be appreciated by those familiar with advanced sigma delta control.Such constraints can be helpful to ensure that the pulsing is performedwithin desired frequency and “on” pulse length parameters. In otherembodiments, more advanced constraints can be imposed by the comparator.For example, if desired, pulse cycle dither 223 can be added to thecomparator.

In some embodiments, it may be desirable to anti-aliasing filter theinput signal 209 and the feedback signal 212. The anti-aliasingfunctionality can be provided as part of the sigma-delta control circuitor it may be provided as an anti-aliasing filter that precedes the sigmadelta control circuit or it may be provided in any other suitable form.In some third order analog continuous time sigma-delta control circuits,the first integrator provides the anti-aliasing functionality. That is,it effectively acts as a low pass filter.

In other embodiments, a variable clock that is based on motor speed maybe used instead of a fixed clock. Such an arrangement isdiagrammatically illustrated in the sigma delta converter of FIG. 8which uses a variable clock that is based on motor speed. Specifically,the clock signal is configured to vary proportionally with motor speed.The use of a variable speed clock when the motor is operating at speedhas the advantage of ensuring that the output of the comparator isbetter synchronized with the motor speed. This, in turn, can helpsimplify the overall design of the converter. The clock can readily besynchronized with the motor speed by utilizing a phase locked loop 229that is driven by an indication of motor speed (e.g., a tachometersignal). A multiplier 231 may then be used to multiply the motor speedsignal 164 to attain the desired sampling rate. The multiplication ofthe motor speed may vary widely based on the needs of any particularsystem. By way of example, frequency multiplication on the order of 10to 1,000,000 times, as for example 10,000 times are appropriate in someapplications. In another example, sigma delta clock rates on the orderof 1 kHz to several hundreds of kilohertz are believed to be suitablefor many automotive applications.

A challenge of using a motor speed based variable clock approach is thatit doesn't work particularly well when the motor is stopped or operatingat particularly low motor speeds. Several different techniques can beused to alleviate such limitations. By way of example, a fixed clock canbe used when the motor is stopped and/or operating a speeds below adesignated idle threshold (e.g., below 600 RPM). In other embodiments, afunctionally intelligent comparator may be used that has specified startand stop routines or switches to a different operating mode during lowspeed operation. In still other embodiments, a non-linear RPM clock maybe used for operations at lower speeds.

There are several ways that the sigma delta converter 200 can beconfigured. In one embodiment (similar to the embodiment illustrated inFIG. 8 ), the input signal 209 is the desired motor duty cycle. In thatembodiment, the feedback signal 212 is the pulsed digital control signal220, which corresponds to pulsed control signal 124 from FIG. 7 . Inthis embodiment, the pulsed control signal 220 represents the desiredmotor duty cycle.

In another embodiment (not shown) the input signal 209 can be consideredto be representative of a desired torque or a desired torque fractionand the feedback signal can be based on the torque output of the motor161 instead of the pulsed digital control signal 220. In such anembodiment, the feedback is more representative of the actual torqueoutput of the motor than the pulsed control signal 220, since itaccounts for any potential torque losses or inefficiencies due toswitching the power supply and motor back and forth between the zero andmost efficient (or other desired) operational states.

In still other embodiments, the feedback signal 212 may be a scaledcombination of the pulsed control signal 220 and the torque output ofthe motor 161. When higher order sigma delta converters are used,differently scaled feedback can be provided to the different integratorsas appropriate for the desired adaptive control using the pulsed controlsignal, the motor torque output or both as the feedback sources.

As suggested above, first order sigma delta converters (like all sigmadelta converters) are helpful in pushing noise to higher frequencies.However, first order sigma delta conversion is not immune to thegeneration of idle tones—which can be the source of unwanted noise orvibration. One way to help minimize or eliminate idle tones is to adddither to the system. Such dither can be added at numerous locations inthe system. In the embodiment illustrated in FIG. 8 optional pseudorandom dither generator 223 (shown in dashed lines) may provide anoptional dither signal 224 as an additional input to the differenceamplifier 201. In other embodiments the dither may instead be injectedat other locations in the sigma delta converter 200, as for example asan additional input to the comparator 205. Higher order sigma deltaconverters are less susceptible to idle tones and therefore there isless of a benefit to adding dither to such systems. Accordingly, ditherwould typically not be used in such systems (although it can be used).

In the embodiments discussed above, pulse width modulation and sigmadelta conversion are used to generate the pulsed control signal. Pulsewidth modulation and sigma delta conversion are two types of convertersthat can be used to represent the input signal. Some of the describedsigma delta converters exhibit oversampled conversion and in variousalternative embodiments, other oversampled converters can be used inplace of sigma delta conversion. In still other embodiments, other typesof converters can be used as well. It should be appreciated that theconverters may employ a wide variety of modulation schemes, includingvarious pulse height or pulse density modulation schemes, code divisionmultiple access (CDMA) oriented modulation or other modulation schemesmay be used to represent the input signal, so long as the pulsegenerator is adjusted accordingly.

As will be appreciated by those skilled in the art, switched reluctancemotors are powerful motors that are relatively inexpensive when comparedto similarly sized induction motors. However, switched reluctance motorstend to be noisy and susceptible to vibrations due to their switching,which make them unsuitable for use in a number of applications. Afeature of sigma delta conversion is its ability to shape noise and topush noise to frequencies that are less (or not) bothersome to humans.As such, controlling switched reluctance motors in a pulsed manner usingsigma-delta or other noise shaping conversion techniques has thepotential to make the use of switched reluctance motors practical in anumber of applications for which they are not currently used.

Managing Transitions

The inherent inductance of the motor can transitorily delay/slow thecurrent/power steps between the on and off motor states. Duringcontinuous (non-pulsed) operation, these transitory effects tend to havea relatively minimal impact on overall motor operation. However, whenrapid pulsing is used as contemplated herein, the transitory effects canhave a larger net impact and therefore, there is more incentive to focuson the motor responsiveness. The nature of the issue will be describedwith reference to FIGS. 9A-9B.

As previously described, a general goal of the pulsed motor control isto operate (power) the motor at its most efficient level for the currentmotor speed during the motor “on” periods and to cut-off power (providezero torque) during the motor “off” periods. Thus, ideally, the powertransitions between the motor power “on” and “off” states would bediscrete steps. This is diagrammatically shown in FIG. 9A whichillustrates the idealized/desired motor drive power for pulsed motorcontrol at a duty cycle of 50%. As seen in FIG. 9A, the transitionsbetween the “on” pulses 302 and the “off” periods 304 are ideally steps.In practice, there are inductive aspects of both the motor and (whenused) the inverter that slow the rise and fall of the power signal. Theactual response of a particular motor will vary widely with theelectrical characteristics of the motor. In general, the motor's actualinput power will rise and fall somewhat exponentially in response to astep change in the commanded motor drive power. The nature of the riseand fall is diagrammatically illustrated in FIG. 9B. As seen therein,there is a ramp-up period (rise time) 306 required for the power signalto actually rise from zero to the desired “on” power level and aramp-down period (fall time) 308 required for the power signal toactually fall from the “on” power level down to zero.

The motor continues to be driven during the power ramp-up and ramp-downperiods. However, the motor operates less efficiently during thoseperiods in a varying manner as can readily be understood with referenceto FIG. 1 . In general, for most given operating speeds, the motorefficiency will drop as the operating power drops from the maximumefficiency line 16 towards zero, with the energy conversion efficiencygetting noticeably worse as the power level approaches zero. Thus, thepulse distortion represented by the power ramp-up and ramp-down periodsdetract from the efficiency gains that can be gained by the describedpulsed operation. In general, the smaller the ratio of the rise/falltimes to the pulse length, the less the transitory switching effectsimpact the motor's energy conversion efficiency during pulsing.

It should be appreciated that the transitory effects shown in FIG. 9Bare shown for the purpose of illustrating the nature of the problem andis not necessarily reflective of the rise/fall times associated withoperation of any particular motor. The relative scale of the rise timeto the pulse length ratio can vary widely based on the available powersupply voltage and the characteristics of the motor used (whichprimarily dictates the rise and fall times), the frequency of thepulsing (which is primarily dictated by the control scheme used) and thepulse width (which is dictated by the control scheme and motor load). Ifthe pulsing is slow compared to the motor response, the rise/fall timesmay be a tiny fraction of the pulse width and the transitory switchingeffects may have a minimal impact on motor performance. Conversely, ifthe pulsing is very rapid and/or the motor response is slow, the ratiothe rise/fall times to the pulse width can become quite significant andcan even exceed the pulse width. If not managed carefully, thetransitory efficiency losses associated with switching can significantlyreduce or even eliminate the theoretical gains that can be attained bypulsed operation. Thus it is important to consider the transitoryswitching effects associated with pulsed operation when determining thepulsing frequency and control schemes that are appropriate for anyparticular application.

A number of techniques can be used to improve the power rise and falltimes. For example, in some embodiments, a resonant capacitor based onmotor inductance is employed. Resonant capacitors can be used to reducethe power rise and fall times by factors of 100 or more (oftensubstantially more), thus they can significantly reduce the transitoryswitching effects associated with pulsed operation. Therefore, it shouldbe appreciated that motors that are designed with pulsed control in mindor modified to improve the transient response of the motor to powerpulses can benefit even more from pulsed operation than existing motors.

In other embodiments, boost converters and/or buck-boost converters maybe used to significantly reduce the rise and fall times associated withswitching between the “on” and “off” motor states. In a particularexample, a boost converter can charge a boost capacitor (sometimesreferred to herein as a kick-start capacitor) to a voltage higher thanthe motor's input voltage. Each time the motor is pulsed on, thekick-start capacitor applies the higher voltage to the motor which canshorten the rise time significantly.

Similarly, a buck-boost converter can be used to charge a buck-boostcapacitor. Each time the motor is pulsed off, the buck-boost capacitorcan store energy from the motor winding's magnetic field, which cansignificantly shorten the pulse's transient fall time

The voltage charge levels and capacitances of the boost and buck-boostcapacitors respectively are chosen appropriately for the motor and itsinductive and resistive characteristics to shorten the transientrise/fall times associated with pulsing the motor on and offrespectively. Preferably, the respective capacitances and charge voltagelevels of the boost and buck-boost capacitors are also selected tomaximize overall motor efficiency during pulsing considering all aspectsincluding inefficiencies associated with the transients themselves andthe effects of any overshoot that may occur due to use of the boost andbuck-boost converters. Since the boost and buck-boost capacitors areused to improve transient response, they may each be opportunisticallyrecharged in the periods between their respective usages—as for exampleduring the motor off periods.

Another factor that is particularly relevant to losses during motor“off” transients relates to the dissipation of energy stored within themagnetic field. In general, there will be an electromagnetic fieldestablished within a motor any time that the motor is operating. Theelectromagnetic field contains a certain amount of energy stored in themagnetic. If the motor is simply turned off, the stored energy willdissipate, which results in loss of the energy that was present withinthe magnetic field. Any such energy losses reduce the overall systemefficiency. Some of that field energy can be recovered by affirmativelycontrolling the off transient of a motor to deliver zero torque duringthe “off” cycles rather than simply cutting the supply of current to themotor to effective turn the motor off. This results in the flow of some“reverse” current from the windings back to/through the power converter140 such that at least some of that energy can be recovered, which meansless energy is lost, thereby improving the system's efficiency.Furthermore, in many applications, managing the power converter 140 todeliver zero torque (as opposed simply turning the power converter off)will result in faster transitions.

Similarly, during the “off” cycles of a generator, the power drawn fromthe generator can be controlled to efficiently manage the capture of thestored energy built up in the motor during the “on” cycles.

FIG. 11 illustrates a power converter/controller that incorporatestransient control circuitry 343 (which may include resonant circuits,boost and buck-boost circuits and/or other transient response improvingcircuits together, alone or in any appropriate combination). In theillustrated embodiment, machine controller 310 directs pulsed control ofpower converter/controller 340—which in turn controls electric machine160. The transient control circuitry 343 is incorporated into the powerconverter 340 itself. In other embodiments, the transient controlcircuitry may be provided as an add-on unit that is placed between thepower converter 340 and the motor/generator or incorporated into themotor/generator itself to accomplish the same functionality (placementnot shown).

Motor Types and Applications

It should be apparent from the foregoing description that the describedpulsed machine control can be utilized in a wide variety of differentapplications to improve the energy conversion efficiency of a widevariety of different types of electric motors and generators. Theseinclude both AC and DC motors/generators.

A few representative types of electric machines that may benefit fromthe described pulsing include both asynchronous and synchronous ACelectric machines including: Induction machines (IM); switchedreluctance machines (SMR); Synchronous Reluctance machines (SynRM);Permanent Magnet Synchronous Reluctance machines (PMaSynRM); HybridPMaSynRMs; Externally Excited AC Synchronous machines (SyncAC)—which arealso sometimes referred to as Electrically Excited Synchronous machines(EESM) or Wound Field Synchronous machines (WFSM) or simply ExternallyExcited Synchronous machines (EESM); Permanent Magnet Synchronousmachines (PMSM); Eddy current machines; AC linear machines; AC and DCmechanically commutated machines; axial flux motors; etc. RepresentativeDC electric machines include brushless, electrically excited, permanentmagnet, series wound, shunt, brushed, compound and others.

Although the structure, control and energy conversion efficiency of thevarious types of electric motors and generators vary significantly, mostelectric machines are designed to operate over a range of operatingconditions and their energy conversion efficiency will vary over thatoperating range—often significantly. In general, the control principlesdescribed herein can be applied to any type of electric machine toimprove the electric machine's efficiency if the electric machine'soperating range includes regions below the equivalent of the maximumefficiency curve illustrated in FIG. 1 . In some circumstancesefficiency gains can be attained by designing an electric machine withpulsed operation in mind.

Some motor designs utilize windings on both the rotor and stator togenerate the motor flux, while others use permanent magnets on eitherthe rotor or stator to contribute to the motor flux. Motors thatincorporate permanent magnets will have flux at zero torque andtherefore will typically have core losses when rotating and produce aback EMF (BEMF) in excess of the supply voltage. In such applications itwill often be desirable to provide a small current to the motor duringthe “torque off” periods in order to maintain zero torque. It should beappreciated that the need to supply current during the “no torque”periods reduces the overall efficiency associated with pulsing andtherefore should be considered when determining which operating rangescan benefit from pulsing. In some operating regions, the lossesassociated with switching and supplying current during the torque offperiods may exceed the efficiency gains associated with pulsing, therebyreducing (or entirely eliminating) the operating range in which pulsedoperation is desirable. However, many electric machines that incorporatepermanent magnets will have operating regions in which the machine'soverall efficiency can be improved through the use of pulsing. Forexample, for Internal Permanent Magnet Synchronous Motors (IPMSM), theoperating region most suitable for pulsed operation are expected to beoperating speeds below (or near to) the threshold speed at which fieldweakening is required.

There is currently widespread interest in using electrical powerplants(e.g., electric motors) in vehicle propulsion systems. Electrics motorsused for vehicle propulsion are commonly referred to as traction motors.In the automotive space there have been significant efforts recently toutilize traction motors alone or in combination with internal combustionengines (hybrids) to drive a vehicle. Today, asynchronous motors andthree phase induction motors are most commonly used in automotiveapplications—both of which are good candidates for the described pulsedmotor control. Automotive applications are notorious for the very widerange of operating conditions that the motor is expected to operateunder—from low speed high torque demands to high speed low torquedemands and everything in between. Under most driving conditions (i.e.,during the significant majority of many drive cycles), the motor isasked to produce far less torque than it is capable of at the currentmotor speed—and indeed most driving occurs in regions where therequested output of the motor is below (often significantly below) themaximum efficiency line 16.

The low load nature of typical driving cycles can be seen in FIG. 10 ,which plots a series of drive points representing the electricpower/torque output of a simulated traction motor/generator running aFederal Test Procedure for a city driving cycle (FTP-75). The drivepoints are plotted on a Torque/Speed/Efficiency graph. As can be seen inFIG. 10 , a significant portion of the drive cycle requires lower torqueas compared to the maximum efficiency curve 16. Thus significantportions of the typical drive cycle are in operating regions that canattain benefits (and often significant benefits) from the describedmotor control approach. FIG. 10 depicts only those portions of the drivecycle where output torque is required. During some portions of the drivecycle regenerative braking may be used with the electric machine actingas a generator. Much of the regenerative braking also occurs at pointswhich can benefit from the pulsed control described herein. It isbelieved that average overall efficiency gains of as much as 7%-12% ormore will be attainable by implementing the described control approachin some automotive applications. Seven to twelve percent betterefficiency translates to 7%-12% more range on the same charge, which hassignificant advantages in the context of automotive applications whererange anxiety is a significant impediment to wide-spread adoption of thetechnology. It is expected that even greater efficiency improvements maybe seen under autonomous driving conditions where there is lessvariability in the requested motor output.

In automotive and other vehicle applications, the operational range ofthe electric motor may be very wide. This is due, in part, to the factthat in most all-electric vehicle applications, the electric motor iscoupled to the driven component(s) with a fixed speed ratio. Thiscontrasts with in internal combustion engine powered vehicle, whichtypically employ an intermediary transmission having variable speedratios between the engine and the driven component(s). As can be seenfairly clearly seen in FIG. 1 , the “sweet spot” of electric motoroperation is often at an intermediate motor speed. When desired,variable gearing can be used to cause the motor to operate in moreefficient regions more of the time. Such gearing can readily be providedby a transmission and thus, there are potential advantages to using atransmission in conjunction with the described pulsed motor control. Thetransmission may have a set of gears or may be continuously variable ormay have any other suitable form. In such embodiments, the motorcontroller or other suitable control component can be arranged to directoperation of the transmission in the desired manner. Again, the use of atransmission can be beneficially employed in a wide variety of other(non-vehicle) related applications as well.

Although automotive applications have been used as an example of avehicle propulsion application, it should be appreciated that thedescribed control approach is equally beneficial in other propulsionrelated applications including: electric motors used in other types ofvehicles including trucks, carts, motorcycles, bicycles, drones andother flying devices; in robots and other devices that move autonomouslywithin an environment; etc.

Motors used in Heating, Ventilation and Air Conditioning (HVAC)applications are another good example of a market that can benefit frompulsed control. There are several factors that contribute to pulsedmotor control being a good fit for HVAC applications. These include thefacts that: (a) the motors used in HVAC applications today arepredominantly induction motors that don't contain permanent magnets; (b)a high percentage of HVAC motors' operational lives are spent inoperating regions below their high efficiency areas; and (c) the inertiaof a fan or pump normally dominates the motor inertia—which tends tofurther mitigate potential NVH related impacts associated with pulsing.

Of course, motors are used in a wide variety of other applications inwhich they are operated at less than their optimal efficiency. This canbe due to operating over a wide operating range (e.g., under a widevariety of different loads and/or motor speeds) or it can be due to theuse of a motor that is oversized (or otherwise not designedspecifically) for its application or any of a variety of other reasons.It should be apparent that the described control approach can bebeneficial to any of these types of applications.

High-Low Torque Modulation

In most of the examples set forth above, pulsing is accomplished bymodulating the torque between a higher (energy efficient) torque outputlevel and a zero torque output level. Although that is believed to bethe preferred approach in most pulsed control application, it isexpected that there will be circumstances (e.g. specificmachines/machine operating region) where it may be preferable tomodulate between higher and lower, non-zero torque outputs rather thanmodulating between high and zero torque. For example, in somecircumstances, High/Low pulsing may have better Noise, Vibration andHarshness (NVH) characteristics that on/off pulsing and thus there maybe circumstances where a more desirable tradeoff between energyconversion efficiency and NVH characteristics may be attained byhigh/low pulsing than by on/off pulsing. In another example, for someoperating regions of some motors, a high/low pulsing approach mayprovide better overall energy conversion efficiency than on/off pulsing.Motors that incorporate permanent magnets that require field weakeningto generate zero torque are particularly good candidates for the use ofhigh-low torque modulation.

Pulsed Motor Overdrive

Most motors have a designated maximum rated output level. Generally, themaximum rated output level is based on steady state operation and oftenthe motor can be driven at higher output levels for brief periods oftime without any adverse effects. In some embodiments, in selectedoperating regions, the output level of motor may be pulsed with the “on”levels being higher than the maximum rated continuous output level forsteady state operation. For some motors in some potential operatingranges, there are several potential advantages to using overdrivepulses. For example, in some specific operating circumstances, theenergy conversion efficiency of the motor or system (e.g., motor andinverter) at a given motor speed may be higher in certain overdriveregions than in “normal” operating regions, which means that pulsedoperation at higher torque or power may be even more efficient.

Furthermore, more efficient operation typically leads to less heating,which potentially facilitates even higher net torque outputs. Thus, itis believed that if motors that are traditionally driven with continuouspower (such as induction and other AC motors, brushless DC motors,switched reluctance motors, etc.) are designed with pulsed operation inmind, they can sometimes be optimized to attain higher net torqueoutputs using pulsed control than would be appropriate using moreconventional steady/continuous drive power.

Other Motor Optimizations

There are a variety of factors that contribute to motor inefficiencies.One contributor relates to the power factor which is the cosine of theangle between the rotating voltage and current vectors. Ideally thevoltage and current should be in phase or have a unity power factor.However for many types of electric motor/generator this ideal does notnecessarily represent the highest system efficiency point for any givenload and speed. When pulsed control of the motor as described herein iscontemplated and the power factor correction is optimized taking intoaccount the pulsed operating points, the effective power factor isexpected to improve above that of traditional continuous motoroperation.

Another factor that contributes to motor inefficiencies is sometimesreferred to as resistive or I²R losses. Resistive losses heat the motorwindings, which in turn further increases resistive losses, sincewinding resistivity generally increases with temperature. Resistivelosses are non-linear—increasing with at least the square of thecurrent. Therefore, resistive losses tend to have a higher impact on theoverall motor efficiency at higher motor output levels—such as thelevels used during pulsed operation. A rule-of-thumb for electric motordesign is that the magnetic losses should approximately equal theresistive losses at the target set point of operation. Using the pulsedmotor control method described herein may influence the design of orchoice of an appropriate motor, since motor operating points below themost efficient operating point will generally not be used. In otherwords, the motor is driven either at substantially its most efficientoperating point or at higher loads. Low load continuous operating neednot be considered in the design or selection of the electric motor—whichagain can help further improve the system overall efficiency.

Another factor that contributes to motor inefficiency is sometimesreferred to as the magnetic core losses—which relates to magnetic fluxoriented losses. One loss mechanism is motor winding leakage reactance,which refers to magnetic flux lines that do not link between rotor andstator magnetic elements. Another magnetic core loss mechanism relatesto hysteresis within magnetic iron cores and are often represented in aBH curve. Here B is the magnetic flux density and H is the magneticfield strength. They are related by the magnetization of the materialsthru which the field passes, which for some motors, is the iron core(s)present in the rotor or stator. Again, a motor that is designedspecifically for pulsed control can be optimized to mitigate magneticcore losses during pulsed motor operation.

As discussed above, transient switching losses associated with switchingbetween motor “on” and motor “off” states during pulsing is anotherfactor that impacts the efficiency of the motor during pulsed operation.As discussed above, one way to reduce these transient switching lossesis to improve (shorten) the motor drive current rise and fall timesassociated with pulsing the motor on and off. Another way to help managethe transient switching losses is to manage the frequency of thepulsing. In general, the lower the switching frequency, the lower thetransient switching losses will be. However there is a tradeoff here inthat lower frequency switching can sometimes induce noise, vibration andharshness (NVH) that may be undesirable or unacceptable in certainapplications. Thus, the pulsing frequency for any particular motor ispreferably selected appropriately considering both motor efficiency andNVH concerns and/or requirements that are relevant to the motorsintended application(s). Along these lines, it is noted the pulsingcontrollers that have noise shaping capabilities such as sigma deltaconversion based pulsing controllers can be very helpful at mitigatingNVH impacts associated with pulsed motor control and can therefore behelpful in supporting the use of generally lower switching frequencies.

It should be appreciated that the appropriate pulsing frequency fordifferent motors may be very different based on the motor'sconstruction, operating environment and operational range. For somemotors, switching frequencies on the order of 10-50 kHz may beappropriate—whereas for other motors much lower switching frequencies,as for example 10-500 Hz range may be more appropriate. Still otherelectric machines may have switching frequencies between these ranges orabove or below either of the stated ranges. The most appropriate pulsingfrequency for any particular motor will depend on a variety of factorsincluding motor size, on/off transient characteristics, NVHconsiderations, etc.

The selection of the desire drive point for any particular motor speedcan also have an impact on the switching frequency. More specifically,many motors have relatively flat efficiency curves over a relativelybroad operational range. In general, pulsed operation at a torque levelthat is slightly lower than the optimal efficiency point of continuousoperation, can sometimes facilitate switching at a slightly lowerfrequency, which—depending on the nature of the switching losses—mayresult in a higher overall motor efficiency during pulsed operation.This emphasizes the point that the desired pulsed operation drive pointassociated with any particular motor speed is not necessarily the torquelevel that would be most efficient for continuous motor operation.Rather, in some circumstances, the most energy efficient point forpulsed operation may be slightly different than the most energyefficient point for continuous operation. Furthermore, NVHconsiderations and/or other operational or control considerations mayaffect the decision as to the drive point that is deemed appropriate forany particular motor speed.

Additional Embodiments

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. The various described pulse controllers and other controlelements may be implemented, grouped, and configured in a wide varietyof different architectures in different embodiments. For example, insome embodiments, the pulse controller may be incorporated into a motorcontroller or an inverter controller or it may be provided as a separatecomponent. Similarly, for a generator, the pulse controller may beincorporated into a generator controller or a rectifier controller andin combined motor/generators the pulse controller may be incorporatedinto a combined motor/generator controller or a combinedinverter/rectifier controller. In some embodiments, the describedcontrol functionality may be implemented algorithmically in software orfirmware executed on a processor—which may take any suitable form,including, for example, general purpose processors and microprocessors,DSPs, etc.

The pulse generator or machine controller may be part of a largercontrol system. For example, in vehicular applications, the describedcontrol may be part of a vehicle controller, a powertrain controller, ahybrid powertrain controller, or an ECU (engine control unit), etc. thatperforms a variety of functions related to vehicle control. In suchapplications, the vehicle or other relevant controller, etc. may takethe form of a single processor that executes all of the requiredcontrol, or it may include multiple processors that are co-located aspart of a powertrain or vehicle control module or that are distributedat various locations within the vehicle. The specific functionalitiesperformed by any one of the processors or control units may be widelyvaried.

The invention has been described primarily in the context of motorcontrol and/or inverter/motor control. However, it should be appreciatedthat the described approach is equally applicable to generator and/orgenerator/rectifier control. Thus, any time that motor control isdescribed it should be appreciated that analogous techniques can beapplied to generator control. Thus, unless the context requiresdifferent interpretation, description of a feature of pulsed motorcontrol, pulsed generator control or pulsed motor/generator controlshould be understood to apply equally to pulsed motor control, pulsedgenerator control and the pulsed control of combined motor/generators.

A variety of different control schemes can be implemented within thepulse controller 120. Generally, the control schemes may be implementeddigitally, algorithmically, using analog components or using hybridapproaches. The pulse generator and/or the motor controller may beimplemented as code executing on a processor, on programmable logic suchas an FPGA (field programmable gate array), in circuitry such as an ASIC(application specific integrated circuit), on a digital signal processor(DSP), using analog components, or any other suitable piece of hardware.In some implementations, the described control schemes may beincorporated into object code to be executed on a digital signalprocessor (DSP) incorporated into an inverter controller (and/orrectifier controller in the context of a generator and/or a combinedinverter/rectifier controller).

In some of the primary described embodiments, sigma delta control isused to create the pulsed control signal. Although sigma delta controlis one particularly good way to create the pulsed control signal 124, itshould be appreciated a variety of other control schemes may be used tocreate the pulsed control signal in other embodiments.

Regardless of the nature of the pulsing that is used, the torquemodulation is preferably managed in a manner such that NVH that isunacceptable for the intended application is not produced.

The described pulsed motor control can be used in a wide variety ofapplications. The biggest efficiency gains will typically be seen inmotors and generators that are not consistently driven at near theiroptimal operating efficiency. A good example of this ismotors/generators that have a wide operational range and are intendedfor use under widely varying load conditions. Another good example ismotors that are routinely under driven. For example, it is not uncommonfor system designers to use larger motors than are actually required foran application—e.g., using a 100 hp motor when a 50 hp motor would bemore than adequate for the assigned tasks. In many cases, the largermotor may run less efficiently at the reduced load and in suchcircumstances, pulsed control may improve the motors efficiency duringuse.

Therefore, the present embodiments should be considered illustrative andnot restrictive and the invention is not to be limited to the detailsgiven herein, but may be modified within the scope and equivalents ofthe appended claims.

What is claimed is:
 1. An electric machine system comprising: anelectrically excited synchronous machine (EESM); a power converter; anda controller arranged receive an indication of a desired electricmachine output and to direct the power converter to cause pulsedoperation of the electrically excited synchronous electric machineduring at least some operating conditions to deliver a net average ofthe desired output at a higher energy conversion efficiency than theelectrically excited synchronous machine would attain when driven incontinuous manner to deliver the desired output.
 2. The electric machinesystem as recited in claim 1 wherein the controller is further arrangedto vary a pulsing frequency of the electrically excited synchronousmachine based at least in part on machine an operating parameter of theelectric machine.
 3. The electric machine system as recited in claim 1wherein the operating parameter is machine speed.
 4. The electricmachine system as recited in claim 1 wherein the controller is furtherarranged to vary a pulsing frequency of the electrically excitedsynchronous machine based at least in part on the desired torque output.5. The electric machine system as recited in claim 1 wherein thecontroller is further arranged to vary a pulsing frequency varies of theelectrically excited synchronous machine based at least in part on noiseor vibration considerations.
 6. The electric machine system as recitedin claim 1 wherein a pulsing frequency of the electrically excitedsynchronous machine is at least 10 Hz.
 7. The electric machine system asrecited in claim 1 wherein the controller includes a sigma deltaconverter configured to determine at least one of a timing of theoutputted pulses, a duration of the outputted pulses, a frequency of theoutputted pulses and a pulse cycle duration of the outputted pulses, thepulse cycle duration being a period between beginnings of sequentialpulses.
 8. The electric machine system as recited in claim 1 wherein thecontroller is configured to turn off the electrically excitedsynchronous machine for at least portions of the times betweensequential pulses.
 9. The electric machine system as recited in claim 8wherein the controller is further configured to turn off the powerconverter for at least portions of the times between sequential pulses.10. The electric machine system as recited in claim 1 wherein thecontroller is configured such that during the pulsed operation of theelectrically excited synchronous machine, an output of the electricallyexcited synchronous machine varies between a first output level andsubstantially zero torque output.
 11. The electric machine system asrecited in claim 1 wherein the controller is further configured to varythe first output level based at least in part on variations in anoperating speed of the electrically excited synchronous machine.
 12. Theelectric machine system as recited in claim 10 wherein the electricallyexcited synchronous machine includes a stator and a rotor and the rotorspins continuously during the pulsed operation of the electricallyexcited synchronous machine.
 13. A wound field synchronous electricmachine (WFSM) comprising: a power converter; and a controller arrangedreceive an indication of a desired electric machine output and to directthe power converter to cause pulsed operation of the wound fieldsynchronous electric machine system during at least some operatingconditions to deliver a net average of the desired output at a higherenergy conversion efficiency than the wound field synchronous electricmachine would attain when driven in continuous manner to deliver thedesired output.
 14. A method of operating an electrically excitedsynchronous machine (EESM), the method comprising; directing pulsedoperation of the electrically excited synchronous electric machineduring at least some operating conditions to deliver a net average ofthe desired output at a higher energy conversion efficiency than theelectrically excited synchronous machine would attain when driven incontinuous manner to deliver the desired output.
 15. The method asrecited in claim 14 further comprising varying a pulsing frequency ofthe electrically excited synchronous electric machine based at least inpart on machine speed during the pulsed operation.
 16. The method asrecited in claim 14 further comprising varying a pulsing frequency ofthe electrically excited synchronous electric machine based at least inpart on noise or vibration considerations.
 17. The method as recited inclaim 14 wherein a sigma delta conversion is utilized to determine atleast one of a timing of the outputted pulses, a duration of theoutputted pulses, a frequency of the outputted pulses and a pulse cycleduration of the outputted pulses, the pulse cycle duration being aperiod between beginnings of sequential pulses.
 18. The method asrecited in claim 14 wherein the electrically excited synchronous machineis turned off for at least portions of the times between sequentialpulses.
 19. The method as recited in claim 18 wherein the powerconverter is turned off for at least portions of the times betweensequential pulses.
 20. The method as recited in claim 14 wherein duringthe pulsed operation of the electrically excited synchronous machine, anoutput of the electrically excited synchronous machine varies between afirst output level and substantially zero torque output.
 21. The methodas recited in claim 14 wherein the first output level varies at least inpart based on variations in an operating speed of the electricallyexcited synchronous machine.
 22. The method as recited in claim 14wherein the electrically excited synchronous machine includes a statorand a rotor and the rotor spins continuously during the pulsed operationof the electrically excited synchronous machine.